The Low Temperature Impact Behavior and Ductile-Brittle Transition of Nanocluster-Strengthened Steel

Caidong Zhang; Yunfei Zhang; Zhiyan Sun; Shuai Ren; Yingli Zhao; Lu Fu; Yan Zhao; Yingfei Wu

doi:10.2320/matertrans.MT-N2023006

Abstract

A series of impact tests within the temperature range of −180∼20°C were applied to study the low temperature impact behavior and ductile-brittle transition of the 700 MPa nanocluster-strengthened steel. The results clearly demonstrated that temperature had a significant effect on impact properties. As the temperature decreased, the impact absorption energy and shear section ratio were correspondingly reduced, and the micro-morphology was changed gradually from ductile fracture to brittle fracture. The ductile-brittle transition temperature (DBTT) range of was between −80°C and −160°C. The fitting results of the Boltzmann function for the ductile-brittle transition curve were in good agreement with the experimental observations. Specifically, the DBTT was determined to be −110 ± 1°C, which suggested good low-temperature impact toughness. The outstanding low-temperature impact toughness of the test steel is mainly due to its low C content and high Ni content, fine effective grain size (EGS), and a large number of Cu-rich nanoscale precipitates. Furthermore, the effect of temperature on dislocation movement plays a major role in the ductile-brittle transition.

Fig. 4 Boltzmann fitting curves of the test steel at different temperatures: (a) impact absorption energy; (b) shear section ratio.

1. Introduction

High strength low alloy (HSLA) steel has a promising application prospect in fields such as shipbuilding and marine platforms due to its low alloy cost, high strength, good plasticity, toughness and low temperature resistance.¹^–⁴⁾ With the development of traditional HSLA steel towards high strength, the carbon content is commonly increased to improve strength, resulting in poor performance in the heat-affected zone (HAZ). For traditional HSLA steel, preheating and post-heat treatment are usually required to ensure the safety of the ship structure.

Cu-rich nanocluster-strengthened steel, as a novel HSLA steel, is strengthened by introducing Cu-rich nanoscale precipitates and fine grain, instead of carbon strengthening. Therefore, compared with traditional HSLA steel, the welding performance of Cu-rich nanocluster-strengthened steel is significantly enhanced. These enhanced properties make it an ideal steel for marine platforms and ships. However, with the rapid development of polar ocean engineering and polar ships, higher requirements have been put forward for the low-temperature toughness of HSLA steels.⁵⁾ The DBTT of materials is an important indicator for evaluating its low-temperature toughness, which could provide guidance for the service environment, service life, the selection of welding materials, and welding parameters.⁶^,⁷⁾ Therefore, it is of great significance to study the DBTT of the steel used in cold regions and polar environments.

Nowadays, researchers have extensively studied the ductile-brittle transition of various metallic materials, such as pure iron, microalloyed steel, and low-carbon martensitic steel.⁸^–¹¹⁾ According to Wang et al.,¹²⁾ increased martensitic steel strength could result in decreased low-temperature toughness; the reason is that a high carbon content can lead to temper brittleness and the formation of coarse carbides. For ferrite-martensite steels, some researchers¹³^–¹⁵⁾ have proved smaller prior-austenite grain size shows better dynamic fracture resistance and lower DBTT. Marmy et al.¹⁶⁾ pointed out the fine tempered martensite structure shows lower DBTT compared to coarser martensite structure, even in neutron irradiated condition. A method for determining the EGS was proposed by Kim et al.,¹⁷⁾ and it was applied in ASTM A508cl.3 bainitic Mn–Mo–Ni low alloy steel. Hanamura et al.¹⁸⁾ utilized EGS to evaluate the ductility and toughness of ultrafine ferrite/cementite steels, specifically analyzing the effects on DBTT of JIS-SM490 steel. The study concluded that the ultrafine ferrite/cementite steel exhibits remarkable toughness thanks to its small EGS value and high fracture surface energy. The correlation of microstructure and Charpy V-notch impact properties of a high-toughness API X70 pipeline steel was investigated by Hwang et al.¹⁹⁾ The results of the study indicate that the decreased energy-transition temperatures observed in steels rolled in the single-phase region can be attributed to the presence of acicular ferrite, which has a smaller EGS. A study conducted on an Nb-microalloyed HSLA steel investigated the impact of five different thermo-mechanically controlled rolled schedules on its microstructure, mesotexture, and fracture behavior.²⁰⁾ The results revealed that the micromechanism of crack initiation and propagation involves the presence of grain boundary carbides and groups of closely aligned grains that act as single effective grains.

Currently, previous researches on HSLA steels suggest that EGS is not the sole factor that affects the ductile-brittle transition behavior of steel under impact loading conditions. Other factors such as precipitates, the distribution of grain boundary misorientation angles, crystallographic texture, and the strength of the steel matrix could also have significant effects on DBTT.²¹^–²³⁾ In this study, a series of impact tests within the temperature range of −180∼20°C were applied to study the ductile-brittle transition behavior of the 700 MPa nanocluster-strengthened steel in depth.

2. Experimental Procedure

The raw material used in this study is 700 Mpa nanocluster-strengthened steel plates with a thickness of 15 mm produced by HBIS Group. The chemical composition is listed in Table 1. The plates were austenitized at 900°C for 1 h followed by water quenching. Then the quenched steel was tempered at 660°C for 1 h followed by air cooling. The low-temperature toughness was determined on a Zwick/Roell 450 J impact tester. The samples with a normative dimension of 10 × 10 × 55 mm and V-notch were impacted at temperatures ranging from −180°C to 20°C. Three parallel samples were tested at each temperature point, and the average values were taken as the impact absorption energy.

Table 1 Chemical composition of the test steel (mass%).

The samples for optical metallography (OM) and scanning electron microscope (SEM) observation were prepared by mechanical polishing and etching (4% nital). EBSD with orientation imaging microscope system was employed on a Hitachi S-3400N SEM to investigate the EGS, grain boundary characteristics. The phase components of the specimens were determined by X-ray diffraction analysis on a PANalytical X’Pert Pro X-ray diffractometer with Cu-Kα radiation. 2θ angles from 20° to 90° were scanned at 40 KV with a step of 0.02°. A Tecnai F30 transmission electron microscope (TEM) was utilized to analysis Cu-rich nanoscale precipitates.

3. Results and Discussions

3.1 Impact toughness

The impact absorption energy, shear section ratio, and micro-morphology of fracture of 700 Mpa nanocluster-strengthened steel at different temperatures are shown in Table 2. In order to analyze and compare the data more intuitively, the data in Table 2 was plotted into the temperature-impact absorption energy curve, as shown in Fig. 1. Table 2 and Fig. 1 show that the impact absorption energy and shear section ratio are reduced with the decrease of temperature. When the temperature decreases from 20°C to −100°C, the impact absorption energy and shear section ratio have little change. The impact absorption energy is above 240 J, and the shear section ratio is above 90%. What’s more, when the temperature is not less than −60°C, the impact absorption energy is about 260 J, and the shear section ratio is 100%. However, the impact absorption energy and section shear ratio of the test steel both considerably fall with the decrease of temperature from −100°C to −120°C. The impact absorption energy drops from 244 J to 45 J (a decrease of 81.6%) and the shear section ratio falls from 94% to 5%, indicating the emergence of the ductile-brittle transition temperature range. As the temperature drops from −120°C to −160°C, the impact absorption energy is reduced to 8 J (a reduction of 82.2%), and the shear section ratio is 0. In addition, with the further decrease of temperature, there is no obvious change in impact absorption energy and shear section ratio.

Table 2 Impact absorption energy, shear section ratio and micro-morphology of fracture at different test temperatures of the test steel.

Fig. 1

Effects of test temperatures on the impact absorption energy of the test steel.

3.2 Morphology of impact fracture

The characteristics of impact fracture morphology can provide insights into the toughness and brittleness of materials. Figure 2 exhibits the macro morphology of impact fracture of the test steel at different temperatures. It is evident that temperature has a substantial impact on the impact fracture morphology. At temperatures between −100°C∼20°C, the macro fracture typically displays ductile behavior, characterized by obvious fibrous zone and large shear lips, shown as Fig. 2(a)∼2(g). As the temperature decreases, the proportion of fibrous zone and shear lips is significantly reduced. As illustrated in Fig. 2(h), the fibrous zone disappears at −120°C and is replaced by the radial region, indicating the changed characteristics of impact fracture. As the temperature further decreases to −160°C, shear lips region is invisible and the macro morphology of impact fracture exhibits a complete brittle fracture with nearly radial zones.

Fig. 2

Macro-morphology of impact fracture surface of the test steel at different temperatures: (a) 20°C; (b) 0°C; (c) −20°C; (d) −40°C; (e) −60°C; (f) −80°C; (g) −100°C; (h) −120°C; (i) −160°C.

The morphology of the impact fracture was further analyzed by SEM, as shown in Fig. 3. When the temperature is between −80°C and 20°C, the microstructure of the fracture mainly consists of dimples with alternating sizes (Fig. 3(a)∼(f)). As the temperature decreases, the size and number of large dimples gradually reduce, as does the average size and depth of dimples, indicating a decrease in toughness. The fracture nevertheless still exhibits fully ductile behavior. When the temperature drops to −100°C, besides dimples with alternating sizes, a small amount of slip appears, as shown in Fig. 3(g), indicating the occurrence of ductile-brittle transition. As the temperature further decreases to −120°C and −160°C, the microfracture morphology exhibits obvious cleavage characteristics. At −120°C (Fig. 3(h)), only a few dimples are observable near the tearing edge, while these dimples become difficult to detect in the microscopic morphology at −160°C (Fig. 3(i)). Additionally, the high and low levels of the cleavage step surface are clearly different in Fig. 3(i), and a small number of secondary cracks can be observed, indicating brittle fracture.

Fig. 3

Micro-morphology of impact fracture surface of the test steel at different temperatures: (a) 20°C; (b) 0°C; (c) −20°C; (d) −40°C; (e) −60°C; (f) −80°C; (g) −100°C; (h) −120°C; (i) −160°C.

3.3 Characterization of DBTT

The temperature at which a material exhibits brittle fracture under external forces is known as its DBTT. A lower DBTT indicates better low-temperature toughness and lower probability of brittle fracture. Various methods are used to characterize DBTT, such as the lateral expansion value, fracture morphology, energy criterion, and Boltzmann function fitting methods. In this study, the Boltzmann function was used to fit the temperature impact absorption energy curve of 700 Mpa nanocluster-strengthened steel, due to its high correlation,²⁴^,²⁵⁾ the Boltzmann function is expressed by eq. (1).²⁶⁾

\begin{equation} A_{KV2} = \frac{A_{1} - A_{2}}{1 + e^{t - t_{0}/t_{k}}} + A_{2} \end{equation}

(1)

where A_kv2 is the impact absorption energy/J, A₁ is the lower platform value/J, A₂ is the upper platform value/J, t is the ambient temperature/°C, t₀ is the DBTT/°C, t_k is the parameter/°C related to the temperature range of ductile-brittle transition. A smaller t_k value corresponds to a narrower temperature range of the DBTT. Thus, a material with a smaller t_k value is more susceptible to transitioning from ductile fracture to brittle fracture.

Figure 4(a) depicts the ductile-brittle transition curve obtained by fitting the temperature-impact absorption energy curve according to eq. (1). According to the fitting results, the test steel’s DBTT is −111.7°C and its lower and upper shelf energies are 7.5 J and 263.1 J, respectively. The correlation coefficient R² of the fitting curve is 0.999, indicating good consistency. Based on the upper and lower shelf energy values, the temperature range for ductile-brittle transition can be determined to be between −80°C and −160°C. Meanwhile, Fig. 4(b) presents the results of fitting the temperature-shear fracture ratio curve using the Boltzmann function. The results demonstrate that the DBTT of the test steel is −109.8°C and the shear fracture ratio of the lower and upper platforms is 0% and 99.5%, respectively. The correlation coefficient R² of the fitting curve is also 0.999, which is a good indication of the consistency. According to the comparison of Fig. 4(a) and Fig. 4(b), the DBTT obtained by fitting the temperature-impact absorption energy and temperature-shear fracture ratio curves are in close agreement, both being within −110 ± 1°C and confirming the ductile-brittle transition range of −80°C to −160°C. Therefore, at temperatures above −80°C, the impact toughness of the steel plate exceeds 250 J, which meets the high toughness requirements of high-strength steel plates for polar ocean engineering and polar ships.

Fig. 4

Boltzmann fitting curves of the test steel at different temperatures: (a) impact absorption energy; (b) shear section ratio.

4. Discussion

4.1 Causes of low-temperature brittleness

When the test temperature falls below DBTT, the sample undergoes a transition from ductile to brittle state, leading to a significant reduction in impact absorption energy. This is a common low-temperature brittle phenomenon, characterized by a change in fracture mechanism from a microporous aggregation type to a transgranular cleavage type, and a transformation of fracture characteristics from fibrous to crystalline. The low-temperature brittleness of materials reflects the plastic deformation ability under low temperatures and high loading rates. The occurrence of low-temperature brittleness in HSLA steel is closely related to changes in yield strength and fracture strength with temperature. Equations (2) and (3) express the fracture strength and yield strength of HSLA steel, respectively, according to the Griffith formula.²⁷⁾

\begin{equation} \sigma_{c} = \sqrt{\frac{2E\gamma_{S}}{\pi a}} \end{equation}

(2)

\begin{equation} \sigma_{s} = \sigma_{i} + k_{y}d^{-1/2} \end{equation}

(3)

Where σ_c is the fracture strength, σ_s is the yield strength, E is the Young’s modulus, γ_s is the plastic work required per unit area for crack propagation, and a is the length of the defect, σ_i is the resistance of dislocation movement within the grain, k_y is the stress concentration coefficient, and d is the grain size. The thermal activation mechanism dominates the movement of dislocations. However, as it has minimal influence on the mechanical conditions for crack propagation, the variation of σ_c with temperature is relatively small. After being aged at 660°C, the crystal structure of test steel is body centered cubic, and the yield strength of this structure is very sensitive to temperature changes. The σ_i increases rapidly as the temperature decreases, leading to a sharp increase in σ_s. As the temperature decreases, brittle fracture occurs when the σ_s equals the σ_i, and the corresponding temperature is DBTT.

When the temperature is above DBTT, the thermal energy of atomic motion is relatively high. The dislocation sources that adjacent to the dislocation pileup groups can be activated by thermal kinetic energy to transfer energy and release stress. When the elastic energy accumulated in the front segment of the dislocation pileup groups is insufficient for cleavage cracks formation, microporous aggregation fracture occurs, which is manifested as obvious plastic deformation on the macro level, with high impact absorption energy and shear section ratio. At temperatures below DBTT, atomic vibrations have relatively small amplitudes and the probability of dislocation sources adjacent to the dislocation pileup group obtaining the energy required for activation through thermal kinetic energy decreases. This makes it difficult to transfer energy and release stress in a timely manner, resulting in a phenomenon known as delayed yield. Therefore, the elastic energy accumulated at the front end of the dislocation pileup group cannot be easily dissipated through the movement of adjacent dislocations. When this energy reaches a critical level, the atom’s bonds are broken, and cleavage fracture occurs along the {100} crystal plane,²⁸⁾ resulting in brittle macroscopic fracture.

4.2 The influencing factors of DBTT

The low-temperature toughness of HSLA steel is highly correlated with the matrix composition and microstructure, which may contribute to various strengthening mechanisms.²⁹⁾ Interstitial solute elements dissolve into the matrix metal lattice and segregate near the dislocation line by interacting with the dislocation to form a Cottrell atmosphere. This not only increases the σ_i but also k_y, ultimately resulting in an increase in σ_s and a decrease in toughness. In medium or high-carbon martensitic steels, the carbon-containing micro-constituents, such as untempered martensite islands, are predominantly brittle sites that initiate brittle fracture.³⁰⁾ The typical microstructure of the test steel is shown in Fig. 5. As can be observed in Fig. 5(a)–(c), the matrix exhibits a mixed structure consisting of lath bainite, lath martensite, and quasi-polygonal ferrite. This is further supported by the XRD results shown in Fig. 6. The test steel does not contain a residual austenite structure that could prevent cleavage fracture.³¹⁾ As is well known, elements added in alloys play an important role in mechanical properties. The alloy element Ni reduces the σ_i and k_y, resulting in σ_s decreases and toughness increases. In addition, Ni can also increase the stacking fault energy and promote the cross slip of screw dislocation at low temperature. The matrix’s high Ni content plays a crucial role in lessening the cleavage tendency and lowering the steel’s DBTT.³²⁾ In this study, the interstitial solid solution element C, increasing the DBTT, is only 0.05 mass%, while the content of Ni, reducing the DBTT, is as high as 1.7 mass%, contributing to the low DBTT of the test steel.

Fig. 5

Microstructure of the test steel: (a) OM maps of transverse section; (b) OM maps of longitudinal section; (c) SEM maps of transverse section; (d) TEM maps of transverse section.

Fig. 6

XRD pattern of the test steel.

Grain boundary orientation has an important influence on crack propagation.³³^,³⁴⁾ Previous investigations have shown that high angle grain boundaries can promote crack deflection while the crack is propagating and impede its extension. The main high angle grain boundaries in HSLA steel are the boundaries of lath blocks.³⁵⁾ The brittle cracks markedly deflect by the grain boundaries with misorientation angles greater than 15°.²⁸⁾ High angle grain boundaries with misorientation angle ≥15°can act as a boundary to separate the crystallization regions, forming the equivalent effective grains.³⁶⁾ As the grain size becomes finer, the dislocation slip distance becomes shorter. Consequently, fewer dislocations accumulate in front of the grain boundary, leading to smaller stress concentrations. At the same time, the total area of grain boundaries increases, reducing the concentration of impurities on the grain boundaries and preventing intergranular brittle fracture from occurring. According to the previous study,²⁸⁾ the DBTT is usually described in inverse proportion to the root square of EGS, as described by eq. (4):

\begin{equation} T_{\textit{DBTT}} = T_{t} - KD^{-1/2} \end{equation}

(4)

Where T_t depends on the tensile properties, K (°C mm^1/2) is a constant that is slightly disparity depending on different microstructure, and D is the EGS. Therefore, refining the EGS can reduce the DBTT and distribute external forces among smaller grains. A fine EGS can slow down cleavage crack spreading in the lower shelf and hinder ductile crack spreading in the upper shelf.³⁷⁾ To investigate the influence of EGS on crack propagation in the test steel, the misorientation map of grains, misorientation angle distribution of grain boundaries were analyzed by EBSD, as shown in Fig. 7. It can be observed that the test steel has a significant number of grain boundaries with large angles. Using the cut-off method, the EGS of the test steel is about 3 µm, which is significantly smaller than the average grain size of the original austenite (11.8 µm). As a result, the small EGS contributes to the test steel’s excellent low-temperature impact toughness.

Fig. 7

EBSD maps of the test steel: (a) grain orientation map; (b) misorientation distributions of grain boundaries.

In the pendulum impact process, the shear section is formed by micro pore aggregation fracture, while the cleavage section is formed by cleavage fracture. The main impact fracture of the test steel at low temperature is cleavage fracture, which generally includes stress concentration, crack initiation and crack propagation. The initiation of cracks is related to the movement and increment of dislocations. Under the effect of impact, the dislocation source is activated, and the impact energy is transmitted inward through the movement and multiplication of dislocations. When dislocations move to the intersection of grain boundaries or the hard second phase, dislocation pileups occurs, and stress concentration forms at the front end of the dislocation pileup group.³⁸⁾ These stress concentration locations are the primary locations where cracks tend to initiate.³⁹⁾ Once the elastic energy generated by stress concentration surpasses the grain boundary strength, intergranular bonding force, or the bonding force between the second phase and the matrix, cracks begin to initiate. For the test steel with high Cu content, Cu completely dissolves in the α-Fe matrix at above the austenitizing temperature. As the temperature decreases, the solubility of Cu in matrix gradually decreases. After aged at 660°C, the supersaturated ε-Cu precipitates out from the ferrite matrix, forming a significant number of Cu-nanoclusters in the matrix of the test steel,⁴⁰⁾ just as Fig. 5 shows. These Cu-nanoclusters interact with dislocations by the precipitate shearing mechanism and Orowan looping mechanism⁴¹^,⁴²⁾ during impact process, resulting in the dissipation of energy transmitted to the sample by pendulum impact. As a result, the dislocation pileup is reduced and crack initiation is suppressed. Therefore, the dispersed Cu-nanoclusters can prevent crack initiation at the precipitate-matrix interface through reducing stress concentrations,⁴³^,⁴⁴⁾ contributing to the high impact toughness of the test steel.

5. Conclusion

In this study, the impact behavior and ductile-brittle transition of the 700 MPa nanocluster-strengthened steel within the temperature range of −180∼20°C was comprehensively investigated. The following conclusions can be formed in light of the results.

(1) The fitting results of the Boltzmann function for the ductile-brittle transition curve were in good agreement with the experimental observations. The ductile-brittle transition temperature range for the test steel is between −80°C and −160°C, and its DBTT is −110 ± 1°C.
(2) The outstanding low-temperature impact toughness of the test steel is mainly due to its low C content and high Ni content, EGS, and dispersed Cu-nanoclusters.
(3) The effect of temperature on dislocation movement plays a major role in the ductile-brittle transition. As the environmental temperature decreases to a critical value, the dislocation movement becomes significantly limited, eventually resulting in cleavage fracture.

REFERENCES

Corresponding author

Version information

Register with J-STAGE for free!